I have a P0678 Code, which as most of you all know is glow plug error on #8 cylinder. I have done the resistance test on the plug and wiring, and the reading on #8 and the other three were all .5. So from what I have read, this points me to the GPCM. But what I don't understand is, how does the ohm test tell you the glow plug is working correctly? Can't the continuity of the wiring and plug be good, but the plug itself still not function properly (i.e. not heat up sufficiently)??

Any insight would be great. What triggers the code to be thrown? I am now just hesitant on what to do. I don't want to spend the 125 or so on the GPCM, just to find it is actually the plug not functioning properly. I also don't want to remove and replace plug (and probably the wiring harness) to find out it indeed was the GPCM. The way I see it there are three options, first this could be a 50 dollar fix (plug and wire harness), second a 125 dollar fix (GPCM), or third a 175 dollar fix (plug, wire harness and GPCM). Really hoping for the first or second, and not getting stuck with the third. (Best would be hidden option four, I save the wire harness and plug only )

One more thing to add, the code does not get thrown every time I start, and it is not consistent. If I clear the code it will sometimes start without throwing the code one to two times before it thrown again. Other times after clearing it throws the code the very next start.

 

Clear DTC then wiggle test the wire try and force a DTC

What you need to do is the pinpoint test

And idk if it's in tech folder or Sean probably posted it on here somewhere do a search

 

Your low ohms reading makes me think GPCM just not putting out enough

You would have high ohms or open circuit if it was plug or wire that gone bad

If you could measure the GPCM Output then tthat might be something

 

My vote is for the GPCM. It been my experience the glow plug fails, CEL comes on. It won't clear the code or reset. One of our Techs have also posted a couple of times that a bad module will sometimes set a code for #8 GP.

All of the GPs I've ever changed read open (no continuity), it a heating coil and must pass current to heat up (and pass a self test). A good glow plug will read 0.5 to 2.0 ohms.

 

Only insight I can give is that I recently had a CEL for a little while that showed a P0678, after I changed the Glow-Plug (actually all 4 on the Left side) & the damn Harness I broke the problem has been resolved.....Good luck